Background: The Trypanosoma cruzi infection is associated with severe T cell unresponsiveness to antigens and mitogens characterized by decreased IL-2 synthesis. Trypanosoma cruzi mucin (Tc Muc) has been implicated in this phenomenom. These molecules contain a unique type of glycosylation consisting of several sialylated O-glycans linked to the protein backbone via N-acetylglucosamine residues. Methodology/Principal Findings: In this study, we evaluated the ability of Tc Muc to modulate the activation of CD4+ T cells. Our data show that cross-linking of CD3 on naı¨ve CD4+ T cells in the presence of Tc Muc resulted in the inhibition of both cytokine secretion and proliferation. We further show that the sialylated O-Linked Glycan residues from tc mucin potentiate the suppression of T cell response by inducing G1-phase cell cycle arrest associated with upregulation of mitogen inhibitor p27kip1. These inhibitory effects cannot be reversed by the addition of exogenous IL-2, rendering CD4+ T cells anergic when activated by TCR triggering. Additionally, in vivo administration of Tc Muc during T. cruzi infection enhanced parasitemia and aggravated heart damage. Analysis of recall responses during infection showed lower frequencies of IFN-c producing CD4+ T cells in the spleen of Tc Muc treated mice, compared to untreated controls. Conclusions/Significance: Our results indicate that Tc Muc mediates inhibitory efects on CD4+ T expansion and cytokine production, by blocking cell cycle progression in the G1 phase. We propose that the sialyl motif of Tc Muc is able to interact with sialic acid-binding Ig-like lectins (Siglecs) on CD4+ T cells, which may allow the parasite to modulate the immune system.